Pivotal bone anchor assembly having twist-in-place insert and receiver with pre-formed axial rotation insert stops

ABSTRACT

A pivotal bone anchor assembly includes a shank with a head and an anchor portion, and a receiver with a channel for receiving a rod and a lower portion of an axial bore for receiving the shank head, with the axial bore including a stop structure and a downwardly-facing surface beneath a helically wound thread. The assembly also includes an insert having an upwardly-facing seat, a lower surface for engaging the shank head, a side structure for engaging the stop structure, and an upwardly-facing surface positioned radially outward from the upwardly-facing seat, with the insert being installed into the axial bore in a first angular position. Subsequent rotation of the insert within the axial bore moves the upwardly-facing surface under the downwardly-facing surface to inhibit upward movement of the insert within the axial bore, with further rotation being inhibited by the side structure coming into engagement with the stop structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application No. 16/745,994,filed Jan. 17, 2020, which is a continuation of U.S. Application No.16/411,826, filed May 14, 2019, now U.S. Pat. No. 10,561,444, which is acontinuation of U.S. Application No. 15/940,343 filed Mar. 29, 2018, nowU.S. Pat. No. 10,335,200, which is a continuation-in-part of U.S.Application No. 15/389,296, filed Dec. 22, 2016, now abandoned, which isa continuation of U.S. Application No. 12/661,042 filed Mar. 10, 2010,now abandoned, which claims the benefit of U.S. Provisional ApplicationNo. 61/210,058 filed Mar. 13, 2009, each of which is incorporated byreference in its entirely herein, and for all purposes.

U.S. Application No. 12/661,042 is also a continuation-in-part of U.S.Application No. 12/229,207, filed Aug. 20, 2008, now U.S. Pat. No.8,353,932, which claims the benefit of U.S. Provisional Application No.60/994,083, filed Sep. 17, 2007, each of which is incorporated byreference in its entirely herein, and for all purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to polyaxial bone screws for use inbone surgery, particularly spinal surgery, and elongate connectingmembers that are at least somewhat plastically deformable. Such screwshave a receiver or head that can swivel about a shank of the bone screw,allowing the receiver to be positioned in any of a number of angularconfigurations relative to the shank.

Many spinal surgery procedures require securing various implants to boneand especially to vertebrae along the spine. For example, elongate orlongitudinal connecting members, such as solid rigid rods are oftenutilized that extend along the spine to provide support to vertebraethat have been damaged or weakened due to injury or disease. Suchelongate members must be supported by certain vertebrae and supportother vertebrae.

The most common mechanism for providing vertebral support is to implantbone screws into certain bones which then in turn support the elongatemember or are supported by the elongate member. Bone screws of this typemay have a fixed head or receiver relative to a shank thereof. In thefixed bone screws, the head cannot be moved relative to the shank andthe rod or other elongate member must be favorably positioned in orderfor it to be placed within the head. This is sometimes very difficult orimpossible to do. Therefore, polyaxial bone screws are commonlypreferred.

Polyaxial bone screws allow rotation of the receiver about the shankuntil a desired rotational position of the receiver is achieved relativeto the shank. Thereafter, a rod or other elongate connecting member canbe inserted into the receiver and eventually the rod and the receiverare locked or fixed in a particular position relative to the shank.

A variety of polyaxial or swivel-head bone screw assemblies areavailable. One type of bone screw assembly includes an open head orreceiver that allows for placement of a rod or other elongate memberwithin the receiver. A closure top or plug is then used to capture therod in the receiver of the screw. Thus, in such bone screws, the closuretop or plug pressing against the rod not only locks the rod in place butalso locks the bone screw shank in a desired angular position withrespect to the receiver. A draw back to such a system occurs when therod or other elongate connecting member is made from a material that ismore flexible and may be more readily deformed or exhibit creep orviscoelastic behavior. Creep is a term used to describe the tendency ofa material to move, flow or to deform permanently to relieve stresses.Material deformation occurs as a result of long term exposure to levelsof stress that are below the yield or ultimate strength of the material.Rods and other longitudinal connecting members made from polymers, suchas polyetheretherketone (PEEK), have a greater tendency to exhibitcreep, than, for example metals or metal alloys. When a rod or otherlongitudinal connecting member exhibits creep deformation over time, theclosure top may no longer tightly engage the connecting member. This initself is not necessarily problematic. However, such loosening alsoresults in loosening of the frictional engagement between the receiverand the bone screw shank that locks the angular orientation of the shankwith respect to the receiver. Body movement and stresses may then resultin undesirable pivoting of the shank with respect to the receivercausing mis-alignment, greater stress and further loosening of thevarious polyaxial bone screw components.

SUMMARY OF THE INVENTION

A polyaxial bone screw assembly of the present invention includes ashank having a generally elongate body with an upper end portion and alower threaded portion for fixation to a bone. The bone screw assemblyfurther includes a receiver having a top portion and a base. The topportion is open and has a channel. The base includes an inner seatingsurface partially defining a cavity and has a lower aperture or opening.The channel of the top portion communicates with the cavity, which inturn communicates with an opening to an exterior of the base. The shankupper portion is disposed in the receiver cavity and the shank extendsthrough the receiver base opening. The cooperating shapes of the shankupper portion external surface and the receiver inner surface enableselective angular positioning of the shank body with respect to thereceiver. The shank upper surface engages a compression insert that inturn engages a longitudinal connecting member being supported within thereceiver. In certain embodiments, the compression insert includes aplanar bottom seat and spaced planar sides for closely receiving anelongate connecting member that has planar sides. Such a compressioninsert can also receive a cylindrical or other shaped connecting member.A single-piece closure structure initially engages the connecting memberand, after some plastic deformation of such member, then the closurestructure engages the compression insert for securing the assembly in awide range of angular orientations.

OBJECTS AND ADVANTAGES OF THE INVENTION

Objects of the invention include: providing an implant wherein all ofthe parts remain together and do not separate; providing a lightweight,low profile polyaxial bone screw that assembles in such a manner thatthe components cooperate to create an overall structure that preventsunintentional disassembly; providing a polyaxial bone screw thatprovides substantially independent locking for the bone screw shank anda deformable longitudinal connecting member; providing such an assemblythat includes a flexible longitudinal connecting member that may be ofnon-circular or circular cross-section; providing such an assembly thatremains in a locked position even if the flexible longitudinalconnecting member undergoes deformation such as creep; providing apolyaxial bone screw with features that provide adequate frictional orgripping surfaces for bone implantation tools and may be readily,securely fastened to each other and to bone; and providing apparatus andmethods that are easy to use and especially adapted for the intended usethereof and wherein the apparatus are comparatively inexpensive to makeand suitable for use.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention and illustrate variousobjects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded front elevational view of a medical implantassembly according to the invention including a polyaxial bone screwhaving a receiver, a bone screw shank, a lower compression insert and aclosure structure cooperating with a longitudinal connecting member inthe form of a bar of non-circular cross-section.

FIG. 2 is an enlarged upper perspective view of the compression insertof FIG. 1 .

FIG. 3 is an enlarged lower perspective view of the compression insertof FIG. 1 .

FIG. 4 is an enlarged top plan view of the compression insert of FIG. 1.

FIG. 5 is an enlarged side elevational view of the compression insert ofFIG. 1 .

FIG. 6 is an enlarged top plan view of the bone screw shank, receiverand the insert being shown during assembly of the insert into thereceiver.

FIG. 7 is an enlarged and partial front elevational view of the bonescrew shank, receiver and insert, also showing the insert in a stage ofassembly similar to FIG. 6 .

FIG. 8 is an enlarged and partial front elevational view of the bonescrew shank, receiver, insert and longitudinal connecting member of FIG.1 shown assembled and with portions broken away to show the detailthereof.

FIG. 9 is an enlarged and partial front elevational view of the bonescrew shank, receiver, insert and longitudinal connecting member of FIG.1 shown assembled and further shown with the closure structure of FIG. 1in an early stage of assembly with the receiver and with portions brokenaway to show the detail thereof.

FIG. 10 is an enlarged and partial front elevational view of the bonescrew shank, receiver, insert and longitudinal connecting member of FIG.1 shown assembled with the closure structure of FIG. 1 and with portionsbroken away to show the detail thereof.

FIG. 11 is an enlarged and partial view similar to FIG. 10 withadditional portions broken away to show the detail thereof.

FIG. 12 is an enlarged and partial upper perspective view of theassembly of FIG. 10 but with the closure structure removed to showdeformation of the longitudinal connecting member and also shown withother portions broken away to show the detail thereof.

FIG. 13 is a perspective view showing the assembly of FIG. 1 attached toa second polyaxial bone screw of FIG. 1 .

FIG. 14 is an enlarged and partial front elevational view of a second,alternative embodiment of a medical implant assembly according to theinvention having a deformable connecting member of rectangularcross-section, with portions broken away to show the detail thereof.

FIG. 15 is an enlarged and partial front elevational view of theembodiment of FIG. 14 with portions broken away to show the detailthereof and further showing replacement of the deformable connectingmember with a rigid rod of circular cross-section.

FIG. 16 is a perspective view showing two bone screws of a third,alternative embodiment of a medical implant assembly according to theinvention holding a deformable connecting member of circularcross-section.

FIG. 17 is an enlarged and partial front elevational view of one of thebone screws and the connecting member of FIG. 16 with portions brokenaway to show the detail thereof.

FIG. 18 is an enlarged and partial front elevational view of theembodiment of FIG. 17 with portions broken away to show the detailthereof and further showing replacement of the deformable connectingmember with a rigid rod of circular cross-section.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure. It is also noted that any reference tothe words top, bottom, up and down, and the like, in this applicationrefers to the alignment shown in the various drawings, as well as thenormal connotations applied to such devices, and is not intended torestrict positioning of bone attachment assemblies of the applicationand cooperating connecting members in actual use.

With reference to FIGS. 1-13 , the reference number 1 generallyrepresents an embodiment of a medical implant assembly according to thepresent invention. The assembly 1 includes a polyaxial bone screw 3having a shank 4 that further includes a threaded body 6 integral withan upper portion 8; a receiver 10; and a lower compression or pressureinsert 12. The medical implant assembly 1 further includes alongitudinal connecting member 14 and a closure structure 16. The shank4, receiver 10, and compression insert 12 are typically factoryassembled prior to implantation of the shank body 6 into a vertebra (notshown).

As will be described in greater detail below, the illustrated shank 4 istop loaded into the receiver 10 and thereafter the substantiallyspherical upper portion 8 slidingly cooperates with an innersubstantially spherical inner surface of the receiver 10 such that thereceiver 10 and the shank 4 can be secured at any of a plurality ofangles, articulations or rotational alignments relative to one anotherand within a selected range of angles both from side to side and fromfront to rear, to enable flexible or articulated engagement of thereceiver 10 with the shank 4 until both are locked or fixed relative toeach other near the end of an implantation procedure. It is noted thatalthough the drawing figures show a top loaded polyaxial mechanismhaving a spherical sliding connection between the shank upper portionand the receiver inner surface, other kinds of top loaded and bottomloaded embodiments may be utilized according to the invention. Forexample, bottom loaded bone screws, such as that disclosed inApplicant’s U.S. Pat. Pub. No. 2007/0055244 (U.S. Pat. App. Ser. No.11/522,503 filed Sep. 14, 2006), the disclosure of which is incorporatedby reference herein, having a threaded capture connection between ashank upper portion and a retainer structure disposed within thereceiver may be utilized for providing a polyaxial connection betweenthe receiver and the shank for use with the present invention.Specifically, U.S. Pat. Pub. No. 2007/0055244 discloses a bone screwshank that includes an upper portion that further includes an outerhelical thread mateable with a retaining structure that includes amating inner helical thread. The retaining structure has a partiallyspherical surface that is slidingly mateable with a cooperating innersurface of the receiver, allowing for a wide range of pivotal movementbetween the shank and the receiver. Bottom or top loaded polyaxial bonescrews with other types of capture connections may also be usedaccording to the invention, including but not limited to other types ofthreaded connections, frictional connections utilizing frusto-conical orpolyhedral capture structures, or other integral top or downloadableshanks.

The shank 4, best illustrated in FIGS. 1 and 9 , is elongate, with theshank body 6 having a helically wound bone implantable thread 24extending from near a neck 26 located adjacent to the upper portion 8 toa tip 28 of the body 6 and extending radially outwardly therefrom.During use, the body 6 utilizing the thread 24 for gripping andadvancement is implanted into the vertebra (not shown) leading with thetip 28 and driven down into the vertebra with an installation or drivingtool, so as to be implanted in the vertebra to near the neck 26. Theshank 4 has an elongate axis of rotation generally identified by thereference letter A.

The neck 26 extends axially upwardly from the shank body 6. The neck 26may be of reduced radius as compared to an adjacent top 32 of thethreaded body 6. Further extending axially upwardly from the neck 26 isthe shank upper portion 8 that provides a connective or captureapparatus disposed at a distance from the threaded body top 32 and thusat a distance from the vertebra when the body 6 is implanted in thevertebra.

The shank upper portion 8 is configured for a polyaxial connectionbetween the shank 4 and the receiver 10 and capturing the shank 4 upperportion 8 in the receiver 10. The upper portion 8 generally includes anouter spherical surface 34; a planar annular upper surface 36 and withan internal drive feature or structure 38 formed in the surface 36. Adriving tool (not shown) has a driving projection configured to fitwithin the tool engagement structure 38 for both driving and rotatingthe shank body 6 into the vertebra. As best shown in FIG. 11 , thespherical surface 34 is also sized and shaped for sliding contactengagement and ultimate positive frictional mating engagement with thecompression insert 12, when the bone screw 3 is assembled, and in anyalignment of the shank 4 relative to the receiver 10. The illustratedsurface 34 also has approximately the same radius as an inner sphericalseating surface (84 described in greater detail below) of the receiver10, allowing for clearance of the shank 4 with respect to the receiver10 and thus a desired degree and magnitude of articulation of the shank4 with respect to the receiver 10. In certain embodiments, the surface34 is smooth. While not required in accordance with the practice of theinvention, the surface 34 may be scored or knurled to further increasefrictional positive mating engagement between the surface 34 and thecompression insert 12.

The shank 4 shown in the drawings is cannulated, having a small centralbore 40 extending an entire length of the shank 4 along the axis A. Thebore 40 is defined by an inner cylindrical wall of the shank 4 and has acircular opening at the shank tip 28 and an upper opening communicatingwith the internal drive 38. The bore 40 is coaxial with the threadedbody 6 and the upper portion 8. The bore 40 provides a passage throughthe shank 4 interior for a length of wire (not shown) inserted into thevertebra (not shown) prior to the insertion of the shank body 6, thewire providing a guide for insertion of the shank body 6 into thevertebra (not shown).

To provide a biologically active interface with the bone, the threadedshank body 6 may be coated, perforated, made porous or otherwisetreated. The treatment may include, but is not limited to a plasma spraycoating or other type of coating of a metal or, for example, a calciumphosphate; or a roughening, perforation or indentation in the shanksurface, such as by sputtering, sand blasting or acid etching, thatallows for bony ingrowth or ongrowth. Certain metal coatings act as ascaffold for bone ingrowth. Bio-ceramic calcium phosphate coatingsinclude, but are not limited to: alpha-tri-calcium phosphate andbeta-tri-calcium phosphate (Ca₃(PO₄)₂, tetra-calcium phosphate(Ca₄P₂O₉), amorphous calcium phosphate and hydroxyapatite(Ca₁₀(PO₄)₆(OH)₂). Coating with hydroxyapatite, for example, isdesirable as hydroxyapatite is chemically similar to bone with respectto mineral content and has been identified as being bioactive and thusnot only supportive of bone ingrowth, but actively taking part in bonebonding.

Referring to FIGS. 1 and 6-12 , the receiver 10 has a generally squaredoff U-shaped appearance with a partially cylindrical inner profile and asubstantially curved or cylindrical outer profile; however, the outerprofile could also be of another configuration, for example, faceted.The receiver has an axis of rotation B that is shown in FIG. 1 as beingaligned with and the same as the axis of rotation A of the shank 4, suchorientation being desirable during assembly of the receiver 10 with theshank 4 and the insert 12. After such assembly, the bone screw 3 isimplanted in a vertebra (not shown). Thereafter, the axis B is typicallydisposed at an angle with respect to the axis A of the shank 4.

The receiver 10 includes a base 50 integral with a pair of opposedsubstantially similar or identical upstanding arms 52 forming asquared-off U-shaped cradle and defining a channel 56 between the arms52 with an upper opening 57 and a lower planar seat 58. The channel 56is defined in part by planar opposed parallel walls 60 of the receiverarms 52 that run perpendicular to the lower planar seat 58. The walls 60are spaced to closely receive the bar-shaped connecting member 14 butmay also receive a cylindrical rod or oval rod having a diameter orwidth the same or less than a width of the connecting member 14.

Each of the arms 52 has an interior surface 64 that defines the innercylindrical profile and includes a partial helically wound guide andadvancement structure 66. In the illustrated embodiment, the guide andadvancement structure 66 is a partial helically wound interlockingflange form configured to mate under rotation with a similar structureon the closure structure 16, as described more fully below. However, itis foreseen that the guide and advancement structure 66 couldalternatively be a square thread, a buttress thread, a reverse anglethread or other thread like or non-thread like helically woundadvancement structure for operably guiding under rotation and advancingthe closure top downward between the arms 52.

Opposed tool engaging apertures 68 are formed on or through surfaces ofthe arms 52 that may be used for holding the receiver 10 during assemblywith the shank 4 and the retainer structure 12 and also during theimplantation of the shank body 6 into a vertebra (not shown).Furthermore, the illustrated embodiment includes upper undercut toolengaging grooves 70 for cooperating with manipulation tools. It isforeseen that tool receiving grooves or apertures may be configured in avariety of shapes and sizes and be disposed at other locations on thereceiver arms 52.

A pair of spring tabs 76, each having an upper body portion 78 integralwith a respective arm 52, and a lower and inner surface 80 extendingbelow the respective upper body portion 78. The surface 80 is sized andshaped for frictional contact with a portion of the insert 12 as will bedescribed in greater detail below. The tabs 76 are generally directedtowards the axis B and downwardly generally toward the base 50. Thelower contact surfaces 80 are positioned to engage the compressioninsert 12 and hold such insert in a desired position, prohibitingrotation of the inert 14 about the axis B. The tabs 76 are typicallyinitially disposed parallel to the axis B and then a tool (not shown) isinserted into the aperture 68 from outside of the receiver 10 to engageand push the respective tab 76, thereby bending the tab 76 inwardly in adirection toward the axis B until the tab 76 is at a desired angularposition, such as is illustrated in FIGS. 7-11 . Such bending of thetabs 76 may be performed either prior to or after assembly of thereceiver 10 with the insert 12. It is also foreseen that the tabs 76 maybe machined or otherwise pre-fabricated to be angled or directed towardthe axis B so as to engage the insert 12 as shown in the drawingfigures. The illustrated tabs 76 are resilient, having a spring-likenature. Thus, when operatively cooperating with the insert 12, the tabs76 bias against the insert 12, holding such insert in a desiredposition; and yet the tabs 76 are flexible enough to allow a user tomake desired minor adjustments of the position of the insert 12 withinthe receiver 10.

With further reference to FIGS. 6-11 , communicating with and locatedbeneath the channel 56 of the receiver 10 is a chamber or cavity,generally 82, defined in part by an internal substantially sphericalseating surface 84 and an inner substantially cylindrical surface 86.The cylindrical surface 86 that defines a portion of the cavity 82 opensupwardly into the channel 56. A closure guide and advancement run-out orrecess 87 is disposed between the guide and advancement structure 66 andthe cylindrical surface 86. The recess 87 is sized and shaped forreceiving a flanged portion of the insert 12 as will be described morefully below. The inner substantially spherical surface 84 that islocated below the surface 86 is sized and shaped for mating with theshank upper portion 8. However, it is noted that the surface 84 couldhave other shapes, for example, conical.

The base 50 further includes a restrictive neck 88 defining a bore,generally 90, communicating with the spherical surface 84 of the cavity82 and also communicating with a lower exterior 92 of the base 50. Thebore 90 is coaxially aligned with respect to the rotational axis B ofthe receiver 10. The neck 88 and associated bore 90 are sized and shapedto be smaller than an outer radial dimension of the shank upper portion8, so as to form a restriction at the location of the neck 88 relativeto the shank upper portion 8 to prohibit the upper portion 8 frompassing through the cavity 82 and out to the lower exterior 92 of thereceiver 10.

With particular reference to FIGS. 2-8 , the lower compression orpressure insert 12 includes a substantially cylindrical body 110integral with a pair of upstanding arms 112. The body 110 and arms 112form a generally squared-off U-shaped, open, through-channel 114 definedby a planar bottom seating surface 116 and opposed spaced planar walls118 that are substantially perpendicular to the seating surface 116. Thelower seating surface 116 and the walls 118 are sized and shaped toconform to a width of the connecting member 14 and thus configured tooperably snugly engage the member 14 at planar outer surfaces thereof aswill be described in greater detail below. The arms 112 disposed oneither side of the channel 114 each include a top flanged portion 120,each portion 120 including a top planar surface 122, sized and shaped toengage the closure structure 16 and partially cylindrical outer surfaces124 sized and shaped to fit within the guide and advancement structurerun-out relief 87 of the receiver 10. The cylindrical surfaces 124 aredisposed substantially perpendicular to the respective adjacent topsurfaces 122. Formed in the planar walls 118 near the top surfaces 122and extending at an oblique angle into the flanged portions 120 are apair of opposed recesses or relief surfaces 126. As will be described ingreater below, the recesses 126 provide relief for material flow of theconnecting member 14 material as shown, for example, in FIGS. 11 and 12. Furthermore, each flange 120 includes a bottom surface 127 disposedsubstantially parallel to the respective top surface 122 and a recessedsurface or groove 128 running at an oblique angle with respect to therespective cylindrical surface 124 removing a portion of the flange 120at the cylindrical surface 124 and the bottom surface 127. The recessedsurface or groove 128 is directed downwardly and inwardly toward thechannel 114, being spaced from the top surface 122 and intersecting thebottom surface 127. Each of the surfaces 128 is sized and shaped toreceive one of the spring tabs 76 of the receiver 10 and engage suchrespective tab at the inner lower surface 80 thereof. As will bedescribed more fully below, after each of the tabs 76 spring or snapinto the respective recessed surface portion 128, the cylindricalsurface 124 located on either side thereof prevents rotation of theinsert 12 about the axis B with respect to the receiver 10.

The compression insert 12 further includes a bottom annular surface 130and a substantially cylindrical outer surface 132. An inner cylindricalsurface 134 partially defines a central through-bore extending along acentral axis of the compression insert 12. The surface 134 is locatedbetween the seating surface 116 and a concave substantially sphericalsurface 136. The compression insert through-bore is sized and shaped toreceive a driving tool (not shown) therethrough that engages the shankdrive feature 38 when the shank body 6 is driven into bone. The surface136 extends between the inner cylindrical surface 134 and the bottomsurface 130. The surface 136 is sized and shaped to slidingly andpivotally mate with and ultimately frictionally engage the outer convexspherical surface 34 of the shank upper portion 8. The surface 136 mayinclude a roughening or surface finish to aid in frictional contactbetween the surface 136 and the surface 34, once a desired angle ofarticulation of the shank 4 with respect to the receiver 10 is reached.A pair of recesses 138 or flat surfaces are formed in the insertcylindrical surface 132 and located spaced from the flanged portions120. With reference to FIG. 14 , such recesses 138 are sized and shapedto engage spring tabs or other insert holding members as will bedescribed in greater detail below.

The cylindrical surface 132 has an outer diameter slightly smaller thana diameter between crests of the guide and advancement structure 66 ofthe receiver 10 allowing for top loading of the compression insert 12with the flanged portions 120 being located between the planar walls 60during insertion of the insert 12 into the receiver 10 as shown in FIGS.6 and 7 . The receiver is then rotated into place as shown in FIG. 8with the flanged portions 120 being received in the guide andadvancement structure run-out or recess 87. As the insert 12 is rotatedinto a desired position, the spring tabs 76 snap into the recessedportions 124, and thereafter hold the insert 12 in a desired alignmentbetween the channel 56 of the receiver and the channel 114 of the insert12. The lower compression insert 12 is sized such that the insert 12 isultimately received within the cylindrical surface 86 of the receiver 10below the guide and advancement structure 66 with the flanged topportions 120 received in the recesses 87 formed below the guide andadvancement structure 66 and the bottom -surface 130 being spaced fromthe receiver base. The receiver 10 fully receives the lower compressioninsert 12 and blocks the structure 12 from spreading or splaying in anydirection. It is noted that assembly of the shank 4 within the receiver10, followed by insertion of the lower compression insert 12 into thereceiver 10 are assembly steps typically performed at the factory,advantageously providing a surgeon with a polyaxial bone screw with thelower insert firmly snapped into place and thus ready for insertion intoa vertebra.

The compression or pressure insert 12 ultimately seats on the shankupper portion 8 and is disposed substantially in the upper cylindricalportion 86 of the cavity 82, with the tabs 76 engaging the insert 12 atthe grooves 128, thereby holding the insert 12 in desired alignment withrespect to the connecting member 14. In operation, the insert 12 extendsat least partially into the channel 56 such that the seating surface 116substantially contacts and engages the adjacent planar surface of theconnecting member 14 when such member 14 is placed in the receiver 10and the closure structure or top 18 is tightened therein. The connectingmember 14 is held in spaced relation with the lower seat 58 of thereceiver 10.

With reference to FIGS. 1 and 10-13 , the elongate connecting member 14illustrated in the drawing figures is a solid elongate bar ofrectangular cross-section. More particularly, the illustrated embodimentis solid and has as square cross-section. Thus, the member 14 includes afirst pair of opposed planar surfaces 140 and a second pair of equallyspaced opposed planar surfaces 142 disposed perpendicular to thesurfaces 140. The illustrated member 14 further includes beveled edgesand first and second end surfaces 144 and 146. The illustratedconnecting member 14 is made from a polymer, in particular,polyetheretherketone (PEEK). The member 14 may be made from a variety ofmaterials including metal, metal alloys or other suitable materials,including, but not limited to plastic polymers such as PEEK,ultrahigh-molecular weight-polyethylene (UHMWP), polyurethanes andcomposites, including composites containing carbon fiber. Furthermore,the connecting member 14 may be a component of a dynamic stabilizationconnecting member, with the bar or bar portion 14 that is operativelydisposed within the insert channel 114 also being integral with orotherwise fixed to a more flexible, bendable or damping component thatextends between adjacent pairs of bone screws 3. It is foreseen that aslong as the longitudinal connecting member has sufficient viscoelasticbehavior, any cross-sectional shape (i.e., square, oval, round,non-round) could be used for the longitudinal connecting member. Thechannel in the insert could be modified to fit the shape of thelongitudinal connecting member. After sufficient pressure is applied tothe longitudinal connecting member by the one-piece closure, and plasticdeformation occurs, additional pressure by the closure is then directlyapplied to the compression or pressure insert, thereby securely lockingboth the longitudinal connecting member and the polyaxial mechanism ofthe bone anchor.

With reference to FIGS. 1 and 9-11 , the closure structure or closuretop 16 can be any of a variety of different types of closure structuresfor use in conjunction with the present invention with suitable matingstructure on the upstanding arms 52. In the embodiment shown, theclosure top 16 is rotatably received between the spaced arms 52. Theillustrated closure structure 16 is substantially cylindrical andincludes an outer helically wound guide and advancement structure 152 inthe form of a flange form that operably joins with the guide andadvancement structure 66 disposed on the arms 52 of the receiver 10. Theflange form utilized in accordance with the present invention may take avariety of forms, including those described in Applicant’s U.S. Pat. No.6,726,689, which is incorporated herein by reference. It is alsoforeseen that according to the invention the closure structure guide andadvancement structure could alternatively be a buttress thread, a squarethread, a reverse angle thread or other thread like or non-thread likehelically wound advancement structure for operably guiding underrotation and advancing the closure structure 16 downward between thearms 52 and having such a nature as to resist splaying of the arms 52when the closure structure 16 is advanced into the channel 56. Theillustrated closure structure 16 also includes a top surface 154 with aninternal drive 156 in the form of an aperture that is illustrated as ahex drive, but may be, for example, a star-shaped internal drive, suchas those sold under the trademark TORX or other internal drives,including, but not limited to slotted, tri-wing, spanner, two or moreapertures of various shapes, and the like. A driving tool (not shown)sized and shaped for engagement with the internal drive 156 is used forboth rotatable engagement and, if needed, disengagement of the closure16 from the receiver arms 52. It is also foreseen that the closurestructure 15 may alternatively include a break-off head designed toallow such a head to break from a base of the closure at a preselectedtorque, for example, 70 to 140 inch pounds. Such a closure structurewould also include a base having an internal drive to be used forclosure removal. A bottom surface 158 of the closure is planar, but mayinclude a point, points, a rim or roughening for engagement with thesurface 140 of the bar-like connecting member 14. The bottom surface 158is sized and shaped for engagement with both the connecting membersurface 140 and the top planar surfaces 122 of the flanged portions 120of the insert 12. As will be described in greater detail below, duringassembly, the surface 158 first engages the surface 140 of theconnecting member. Then, as the closure member 16 is rotated, thesurface 158 presses against the surfaces 120, pushing the insert 16downwardly onto the shank upper portion 8 that in turn presses againstthe receiver surface 84, locking the shank 4 with respect to thereceiver 10 in a desired angular or articulated position. With time, theconnecting member 14 may undergo creep or other plastic deformation thatmay lessen the engagement between the surfaces 140 and 158. However,regardless of any movement of the surface 140, the frictional engagementbetween the closure member 16 and the insert 12, both preferably madefrom a metal or metal alloy, such as stainless steel or titanium, willremain rigid and secure.

The closure top 16 may further include a cannulation through boreextending along a central axis thereof and through a surface of thedrive 156 and the bottom surface 158. Such a through bore provides apassage through the closure 16 interior for a length of wire (not shown)inserted therein to provide a guide for insertion of the closure topinto the receiver arms 52.

With particular reference to FIG. 1 , prior to the polyaxial bone screw3 being placed in use according to the invention, the tip 28 of theshank 6 is inserted into the receiver 10 at the upper opening 57 andthen through the bore 88 to a position wherein the shank upper portion 8is seated on the inner surface 84 of the receiver. Then, with particularreference to FIGS. 6-7 , the insert 12 is inserted into the opening 57with the flanged portions 120 aligned in the channel 56, each flangedportion 120 being located between a pair of opposed planar walls 60partially defining the channel 56. The insert 12 is then moveddownwardly in the channel 56 and toward the cavity 82 as illustrated bythe arrow 170 in FIG. 7 . As the insert 12 is moved downwardly into thecylindrical portion 86 of the cavity 82, the spring tabs 76 may bepushed outwardly away from the axis A by the flanged portions 120. Oncethe flanged portions 120 are located below the guide and advancementstructure 66 and adjacent the run-out relief 87, the insert 12 isrotated about the axis B of the receiver 10 as illustrated by the arrow172 in FIG. 8 . The flanged portions 120 fit within the relief 87. Onceeach flanged portion 120 is located centrally with a respective arm 52of the receiver 10, rotation is ceased and the spring tabs 76 slide orsnap into the grooves 128. A slight downward movement of the insert 12may be needed to fully engage the spring tabs 76 in the grooves witheach of the surfaces 80 being biased against the respective groovesurfaces 128. The insert 12 is now locked into place inside the receiver10 with the guide and advancement structure 66 prohibiting upwardmovement of the insert out of the opening 57 and the spring tabs 76 thatare biasing against the insert 12 at the grooves 128 prohibitingrotational movement of the insert 12 with respect to the receiver 10about the receiver axis B. As illustrated in FIG. 10 , the insert 12seats on the shank upper portion 8 with the surface 136 in slidingengagement with the surface 34. The run-out or relief 87 is sized andshaped to allow for some upward and downward movement of the insert 12toward and away from the shank upper portion 8 such that the shank 8 isfreely pivotable with respect to the receiver 10 until the closurestructure 16 presses on the insert 12 that in turn presses upon theupper portion 8 into locking frictional engagement with the receiver 10at the surface 84.

In use, the bone screw 3 is typically screwed into a bone, such as avertebra (not shown), by rotation of the shank 4 using a driving tool(not shown) that operably drives and rotates the shank 4 by engagementthereof with the tool engagement structure 38. The vertebra (not shown)may be pre-drilled to minimize stressing the bone and have a guide wire(not shown) that is shaped for the cannula 40 inserted to provide aguide for the placement and angle of the shank 4 with respect to thevertebra. A further tap hole may be made using a tap with the guide wireas a guide. Then, the bone screw 3 is threaded onto the guide wireutilizing the cannulation bore 40 by first threading the wire into thebottom opening 28 and then out of the top at the internal drive 38. Theshank 4 is then driven into the vertebra, using the wire as a placementguide.

With reference to FIGS. 10-11 , the connecting member 14 is eventuallypositioned in an open or percutaneous manner within the receiver channel56 and then into the channel 114 defined by the bottom planar seatingsurface 116 and the planar walls 118. The member surfaces 140 and 142are closely received within the planar walls of the insert 12. Theclosure structure or top 16 is then inserted into and advanced betweenthe arms 52 so as to bias or push against the upper surface 140 of theconnecting member 14. Alignment of the planar surfaces 140 and 142 ofthe connecting member 14 with the squared off U-shaped channel 114 ofthe insert 12 is initially provided and then maintained by pressureplaced on the insert 12 at grooves 128 by the spring tabs 76. Theclosure structure 16 is rotated, using a tool engaged with the innerdrive 156 until a selected pressure is reached at which point theconnecting member 14 fully engages the planar surfaces 116 and 118 ofthe insert 12 and the connecting member 14 is urged toward, but not incontact with the lower seat 58 of the receiver 10 that defines thesquared off U-shaped channel 56. As the closure member 16 is rotated andurged downwardly against first the connecting member 14 and then theflanged portions 120 of the insert 12, for example, with a pressure ofabout 80 to about 120 inch pounds, frictional locking of the shank upperportion 8 against the receiver surface 84 at a desired angle ofarticulation is accomplished not only by forces transferred through theconnecting member 14 but also by direct engagement between the closuremember 16 and the insert 12 at the flanged portions 120. Thus, if theconnecting member 14 exhibits creep, as would be expected by the PEEKconnecting member 14 illustrated in the drawing figures, movement orflow of the member 14 would not diminish the locking frictionalengagement between the shank upper portion 8 and the receiver surface 84as neither the insert 12 nor the receiver 10 (both made from metal suchas titanium, for example) would exhibit creep or other deformation. Insuch an assembly 1, the benefit to the patient of a flexible or dynamicconnecting member 14 as well as the benefit of a bone screw 3 having asecure locking mechanism (metal to metal frictional engagement) isaccomplished.

If removal of the connecting member 14 from any of the bone screws 3 isnecessary, or if it is desired to release the connecting member 14 at aparticular location, disassembly is accomplished by using the drivingtool (not shown) that mates with the internal drive 156 on the closurestructure 16 to rotate and remove the closure structure 16 from thecooperating receiver 10. Disassembly is then accomplished in reverseorder to the procedure described previously herein for assembly.

With reference to FIG. 12 , there is illustrated an assembly 1 of theinvention wherein the closure member 16 has been removed after an amountof time wherein the PEEK connecting member 14 has exhibited somedeformation due to creep. It is noted how the member 14 is compressed atthe area 180 where the closure structure 16 bottom surface 158 had beenpressing on the member upper surface 140. Also illustrated is the flowof connecting member 14 material 182 into the recesses 126 formed in theinsert 12. Such material 182 disposed within the recesses 126advantageously provides further frictional engagement between the insert12 and the connecting member 14.

With reference to FIG. 13 , the connecting member 14 is typicallyassembled with two or more bone screws 3. The combination of theconnecting member 14 with planar surfaces and the bone screw receiver 10having a channel and insert 12 that includes planar surfaces for closelyreceiving the member 14 is shown. An advantage of such an assembly istorsional control of the medical implant system. As compared to rigidrods made from metals or metal alloys, a dynamic medical implant 1 ofthe invention is desirably more flexible in bending or flexing.Furthermore, the combination between a bar-shaped connecting member andreceiver with planar surfaces provides stability and strength towithstand torsional forces that, for example, a cylindrical PEEK rodcaptured by a receiver with a U-shaped channel would not provide. If amore rigid support is eventually required, the bar-shaped member 14 maybe replaced by a stiffer cylindrical or bar-shaped rod having a diameteror width the same or similar to the cross-sectional width of the member14. Such a rod of circular cross-section would be adequately receivedand closely held between the planar walls 116 and 118 of the insert 12and the same or similar closure top 16 could be used to hold such a rodin the receiver 10 and also lock the polyaxial mechanism, placing theshank 4 and the receiver 10 in a desired angular relationship with oneanother.

With reference to FIGS. 14 and 15 , an alternative assembly 1′ isillustrated. The assembly 1′ is identical to the assembly 1 previouslydescribed herein with the exception of an aspect of a receiver 10′ thatis otherwise substantially similar to the receiver 10 previouslydescribed herein. The assembly 1′ therefore includes a shank 4′, aninsert 12′, a connecting member 14′ and a closure member 16′ that areidentical or substantially similar in form and function to therespective shank 4, insert 12, connecting member 14 and closure member16 previously described herein with respect to the assembly 1 Ascompared to the spring tabs 76 of the receiver 10 that extend in adownward direction toward the base 50 of the receiver 10, the receiver10′ includes a pair of spring tabs 76′ that extend upwardly and toward aclosure structure 16′. The spring tabs 76′ bias against the insert 12′at recesses 138′ identical to the recesses 138 described herein withrespect to the insert 12.

With reference to FIG. 15 , if a more rigid support is eventuallyrequired, the bar-shaped member 14′ is shown being replaced by a stiffercylindrical rod 14″ having a diameter equal to the width of the member14′. As shown in FIG. 15 , the rod 14″ is received and closely heldbetween planar walls 116′ and 118′ of the insert 12′ and a closure top16″ substantially similar to the closure top 16′ abuts against the rod14″ but does not abut against the lower pressure insert 12′. Frictionalengagement of the closure top 16″ and the rod 14″ fixes the rod 14″ inthe receiver 10′ and also locks the polyaxial mechanism, fixedly placingthe shank 4′ and the receiver 10′ in a desired angular relationship withone another.

With reference to FIGS. 16-18 , another alternative assembly 201 isillustrated. The assembly 201 is identical to the assembly 1′ previouslydescribed herein with the exception that the bottom planar surfaces ofthe lower pressure insert and receiver have been replaced by curvedsurfaces, forming U-shaped channels for holding a connecting memberhaving substantially circular cross-section, such as deformable or rigidrods. The assembly 201 therefore includes a shank 204, a receiver 210,an insert 212, a connecting member 214 and a closure member 216 that aresubstantially similar in form and function to the respective shank 4,receiver 10, insert 12, connecting member 14 and closure member 16previously described herein with respect to the assembly 1 with thefollowing exceptions: The receiver 210 includes spring tabs 276 that areidentical or substantially similar to the spring tabs 76′ of theassembly 1′. Also, a U-shaped surface 258 replaces the planar surface 58that partially defines the channel 56 of the receiver 10. Similarly, theinsert 212 includes a U-shaped surface 316 that replaces the planarbottom surface 116 and portions of the side surfaces 118 of the insert12. Therefore, the insert 212 and the receiver 210 are sized and shapedto closely receive the connecting member 214 that differs from theconnecting member 14 in that the member 214 has a circular cross-sectionas compared to the rectangular cross-section of the member 14. As bestillustrated in FIG. 17 , the rod-shaped deformable connecting member 214is closely held or cradled by the insert 212 surface 316; with theconnecting member 214 being held spaced from the receiver surface 258.The closure top 216 presses and deforms the connecting member 214 andalso engages the lower pressure insert 212. Engagement between theclosure 216 and the insert 212 keeps the bone screw shank 204 in adesired locked position with respect to the-receiver 210 even if furtherdeformation of the connecting member 214 occurs that might loosen theconnection between the connecting member 214 and the closure top 216

With reference to FIG. 18 , if a more rigid support is eventuallyrequired, the deformable rod 214 is shown being replaced by a stiffercylindrical rod 214′. The more rigid rod 214′ is received and closelyheld by the surface 316 and a closure top 216′ substantially similar tothe closure top 216 abuts against the rod 214′ but does not abut againstthe lower pressure insert 212. Frictional engagement of the closure top216′ and the rod 214′ fixes the rod 214′ in the receiver 210 and alsolocks the polyaxial mechanism, fixedly placing the shank 204 and thereceiver 210 in a desired angular relationship with one another.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

What is claimed is:
 1. A pivotal bone anchor assembly for securing anelongate rod to a bone of a patient via a closure top, the elongate rodincluding an underside portion, the pivotal bone anchor assemblycomprising: a receiver comprising a longitudinal axis, a base defining acavity communicating with a bottom surface of the base through a bottomopening and including at least a partial spherical seating surfaceproximate the bottom opening, and an upper portion defining a firstchannel extending transverse to the longitudinal axis configured toreceive the elongate rod, the cavity communicating with the firstchannel to define an axial bore centered about the longitudinal axis andextending upward from the bottom opening through the first channel to atop of the upper portion, the axial bore including a lower cylindricalsurface in the cavity above the at least partial spherical seatingsurface, an at least partially cylindrical upper surface in the firstchannel including a discontinuous helically wound thread formed therein,and opposed alignment and holding members protruding inwardly beneaththe discontinuous helically wound thread; a shank comprising a shankhead at a proximal end configured for top loading within the receiverand an anchor portion opposite the shank head configured for fixation tothe bone, the shank head including a partial spherical shape and aninternal drive structure, the partial spherical shape of the shank headpivotally engageable with the at least partial spherical seatingsurface; and a pressure insert comprising a centerline axis, a secondchannel formed between a pair of upright arms configured to receive atleast the underside portion of the elongate rod, a central opening alongthe centerline axis configured for tooling to pass through to engage theinternal drive structure of the shank head, a lower surface configuredto engage the shank head to apply downward pressure directly on theshank head, the upright arms including an outer cylindrical surface withopposite radially outward extending flanges across at least a portion ofa circumference of each outer cylindrical surface and formed at a topthereof, each flange including a notch formed thereon, the pressureinsert configured for top loading and installation into a first positionwithin the axial bore with the centerline axis of the pressure insertco-axially aligned with the longitudinal axis of the receiver; whereinthe opposed alignment and holding members protrude inwardly into theaxial bore of the receiver prior to the pressure insert being installedinto the first position and the second channel of the pressure insert isout of alignment with the first channel of the receiver, and whereinupon rotation of the pressure insert about the longitudinal axis of thereceiver into a second position within the axial bore, the opposedalignment and holding members are positioned within the notches formedon the opposite radially outward extending flanges so has to hold thesecond channel of the pressure insert in alignment with the firstchannel of the receiver.
 2. The pivotal bone anchor assembly of claim 1,wherein the opposed alignment and holding members further comprise apair of resilient spring tabs.
 3. The pivotal bone anchor assembly ofclaim 1, wherein the shank head is configured for positioning into thecavity of the receiver prior to the pressure insert being installed intothe first position within the axial bore.
 4. The pivotal bone anchorassembly of claim 1, wherein the opposite radially outward extendingflanges each include a top surface defining an upwardly-facing planarsurface, and wherein upon rotation of the pressure insert into thesecond position, each flange is positioned under a downwardly-facingsurface formed into the axial bore beneath the discontinuous helicallywound thread and configured to at least partially overlap and beengageable with the upwardly-facing planar surface.
 5. The pivotal boneanchor assembly of claim 4, wherein each of the opposite radiallyoutward extending flanges project radially outwardly from upper portionsof the upright arms with the top surfaces of the flanges furtherdefining top surfaces of the upright arms.
 6. The pivotal bone anchorassembly of claim 1, wherein the shank is cannulated with an axialopening extending along at least a substantial length of the anchorportion.
 7. The pivotal bone anchor assembly of claim 1, wherein thediscontinuous helically wound thread comprises one of a discontinuoussquare thread, a buttress thread, or a reverse angle thread.
 8. Thepivotal bone anchor assembly of claim 1, wherein the pressure insert hasa top surface that is spaced apart from a bottom surface of the closuretop when the closure top is fully engaged with the discontinuoushelically wound thread of the axial bore and the insert is in the secondposition within the axial bore.
 9. The pivotal bone anchor assembly ofclaim 1, wherein the receiver includes a pair of upright arms extendingupwardly from the base, wherein the first channel is locate between thepair of upright arms, and the pair of upright arms includessymmetrically opposite, radially outward opening, horizontally extendinginstrument engaging grooves.
 10. A pivotal bone anchor assembly forsecuring an elongate rod to patient bone via a closure top, the elongaterod including an underside portion, the pivotal bone anchor assemblycomprising: a receiver including a longitudinal axis, a base, and anintegral upper portion defining a channel extending transverse to thelongitudinal axis configured to receive the elongate rod, the base incommunication with a bottom surface of the base through a bottomopening, the receiver further including an axial bore centered about thelongitudinal axis and extending from the bottom opening to a top of theintegral upper portion, the axial bore defined in part by the bottomopening and an at least partially cylindrical inner surface in thechannel including a discontinuous helically wound thread formed therein,the axial bore including an insert-engaging stop beneath thediscontinuous helically wound thread and a downwardly-facing surfacepositioned beneath the discontinuous helically wound thread; a shankincluding a proximal at least partially spherical shaped head and anintegral anchor portion extending distally from the head for fixation tothe bone, the head being positionable within the axial bore of thereceiver with the shank extending downwardly through the bottom opening,the head including an internal drive structure; a pressure insertincluding a longitudinal axis, an upwardly-facing seat configured toreceive at least the underside portion of the elongate rod, a centraltool passage opening through the upwardly-facing seat allowing accesstherethrough for tool engagement of the internal drive structure of thehead, a lower surface configured to directly engage the head to applydownward pressure to the head, at least one side structure configured toengage the insert-engaging stop in the axial bore, and anupwardly-facing surface positioned radially outward from theupwardly-facing seat, the pressure insert being installed into the axialbore in a first position, wherein rotation of the pressure insert aboutthe longitudinal axis of the receiver from the first position to asecond position within the axial bore causes the upwardly-facing surfaceto be rotated under the downwardly-facing surface so as to inhibitupward movement of the pressure insert within the axial bore of thereceiver along the longitudinal axis of the receiver, further rotationof the pressure insert about the longitudinal axis of the receiver beinglimited by the at least one side structure coming into engagement withthe insert-engaging stop.
 11. The pivotal bone anchor assembly of claim10, wherein the insert-engaging stop further comprises a pair of opposedinwardly-protruding integral structures formed into opposite sides ofthe axial bore of the receiver.
 12. The pivotal bone anchor assembly ofclaim 10, wherein the upwardly-facing seat of the insert is furtherdefined and flanked by a pair of upright arms.
 13. The pivotal boneanchor assembly of claim 10, wherein the pressure insert has a topsurface that is spaced apart from a bottom surface of the closure topwhen the closure top is fully engaged with the discontinuous helicallywound thread of the receiver in securing the elongate rod within thechannel of the receiver and the insert is in the second position withinthe axial bore.
 14. The pivotal bone anchor assembly of claim 10,wherein the pressure insert is top loaded into the channel of thereceiver when installed in the first position.
 15. The pivotal boneanchor assembly of claim 10, wherein the lower surface of the insertfurther comprises a downwardly-facing spherical surface.
 16. The pivotalbone anchor assembly of claim 10, wherein the shank head is installedinto the receiver prior to the pressure insert being installed into theinstalled into the axial bore.
 17. The pivotal bone anchor assembly ofclaim 10, wherein the shank is cannulated with an axial openingextending along at least a substantial length of the anchor portion. 18.The pivotal bone anchor assembly of claim 10, wherein the discontinuoushelically wound thread comprises one of a discontinuous square thread,buttress thread, or reverse angle thread.
 19. The pivotal bone anchorassembly of claim 10, wherein when the pressure insert is installed intothe axial bore in a first position, the upwardly-facing seat of theinsert is out of alignment with the channel of the receiver.
 20. Amethod of assembling a pivotal bone anchor assembly for securing anelongate rod to patient bone via a closure top, the method comprising:top loading a shank into a receiver and positioning a head of the shankwithin an axial bore of the receiver with the shank extending downwardlythrough a bottom opening of the receiver, wherein the shank includes ananchor portion extending distally from the head for fixation to thebone, the head being on a proximal end of the shank and at leastpartially spherical shaped, the head including an internal drivestructure; top loading a pressure insert into the receiver subsequent tothe top loading of the shank into the receiver, the pressure insertbeing installed into the axial bore in a first position where anupwardly-facing seat of the insert is out of alignment with a channel ofthe receiver, the insert including a longitudinal axis, a central toolpassage opening through the upwardly-facing seat allowing accesstherethrough for tool engagement of the internal drive structure of thehead, a lower surface configured to directly engage the head to applydownward pressure to the head, at least one side structure configured toengage an insert-engaging stop in the axial bore of the receiver, and anupwardly-facing surface positioned radially outward from theupwardly-facing seat, the upwardly-facing seat configured to receive atleast an underside portion of the elongate rod; rotating the pressureinsert about a longitudinal axis of the receiver from the first positionto a second position within the axial bore to cause the upwardly-facingsurface to be rotated under a downwardly-facing surface of the axialbore of the receiver so as to inhibit upward movement of the pressureinsert within the axial bore of the receiver along a longitudinal axisof the receiver, further rotation of the pressure insert about thelongitudinal axis of the receiver being limited by the at least one sidestructure coming into engagement with an insert-engaging stop of theaxial bore of the receiver, wherein the receiver includes a base, and anupper portion defining the channel extending transverse to thelongitudinal axis of the receiver, the channel configured to receive theelongate rod, the base in communication with a bottom surface of thebase through the bottom opening, the axial bore centered about thelongitudinal axis of the receiver and extending from the bottom openingto a top of the upper portion, the axial bore defined in part by thebottom opening and an at least partially cylindrical inner surface inthe channel including a discontinuous helically wound thread formedtherein, the axial bore including the insert-engaging stop beneath thediscontinuous helically wound thread and the downwardly-facing surfacepositioned beneath the discontinuous helically wound thread.